Print through elimination in fiber reinforced matrix composite mirrors and method of construction

Abstract

A fiber reinforced matrix composite mirror that eliminates the problem of “print through”. A layer of small unbundled fibers in the matrix diffuses and randomize any stresses that are created by the weave pattern of the fiber reinforcement so that the coarse texture is not transferred to the optical quality surface thus eliminating “print through”. The layer can be provided in a variety of embodiments using random fibrils, a continuous fiber mat or a weave of single or finely towed continuous fibers. The fiber reinforced mirror is constructed by adding a mixture of matrix pre-cursor and fibers or fibrils to a common graphite fiber reinforced matrix construction.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] This invention relates to fiber reinforced mirrors, and more specifically to the elimination of “print through” in fiber reinforced mirrors. [0003] 2. Description of the Related Art [0004] Mirrors have been utilized in high precision optical systems for many decades. Critical to the successful application of these mirrors are material properties such as dimensional stability, low weight, high thermal conductivity, high stiffness, low coefficient of thermal expansion, etc. Different applications require various combinations of these properties. For example, many aerospace, telescope, airborne optics and fast-scanning optical applications require low weight, high stiffness and high dimensional stability. Beryllium is particularly attractive in this regard. [0005] Beryllium mirrors are fabricated by consolidating beryllium powder by hot isostatic pressing (HIP) into a block, which is then machined into the desired shap...

AI Technical Summary

Benefits of technology

[0010] The present invention provides a fiber reinforced matrix composite mirror and method of construction that eliminates the problem of “print through”.

[0011] This is accomplished by forming a layer of small unbundled fibers within the matrix on the surface of the fiber reinforced substrate. Although this layer contributes to the overall mirror stiffness, its primary function is to diffuse out and randomize any stresses that are created by the weave pattern of the fiber reinforcement so that the coarse texture is not transferred to the optical quality surface, thus eliminating “print through”.

[0013] In an exemplary embodiment, the mirror includes a carbon-carbon substrate (a stack of graphite fiber weaves in a carbon matrix) and a layer of submicron graphite fibrils bound within the same carbon matrix. An optical quality surface is formed either in the fibril layer or in another thin layer formed of a metal, semi-metal or ceramic material. The graphite fibrils have substantially the same CTE and stiffness properties as the graphite fiber weave. The carbon matrix does not have the strength and stiffness of the fibers but does exhibit a similar CTE. The fibrils have a submicron diameter, preferably less than 0.3 microns with a length greater than 100 times their diameter and provide a very fine structure for diffusing any pattern of stress.

Problems solved by technology

Beryllium is a very expensive metal and has limited sources of availability.

Furthermore, beryllium dust is toxic and has very limited near-net-shaping capability.

A high precision beryllium mirror is therefore machined out of a beryllium block, thus wasting most of the beryllium by converting it into beryllium chips.

The toxicity of the beryllium dust requires a special machine shop to meet rigid Occupational Safety and Health Administration (OSHA) requirements for safety, which adds to its expense.

The un-reinforced layer does not contribute appreciably to the strength or stiffness of the composite mirror.

This is known as “print through” and effectively degrades the optical properties of the mirror.

As a result, fiber reinforced mirrors have not achieved successful commercialization to replace beryllium and other isotropic metal mirrors.

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